1. Industrial Field of the Invention
The present invention relates to an optical head of an optical disk apparatus for recording or reproducing information optically in an optical disk.
2. Related art of the Invention
The objective lens used in an optical head is designed in consideration of the thickness of an optical disk, and in an optical disk having a thickness different from the design value, spherical aberration occurs and the converging performance deteriorates, making recording or reproducing difficult. Hitherto, the compact disc (CD), video disc and magneto-optical disk for data were all same in the thickness of 1.2 mm. It was hence possible to record and reproduce different types of optical disks by one optical head.
Recently, however, in order to heighten the density of optical disk, it has been studied to increase the number of apertures of the objective lens. When the number of apertures of the objective lens is increased, the optical resolution is improved, and the frequency band for recording and reproducing can be expanded, but if there is an inclination in optical disk, coma aberration increases. Due to warp of the optical disk and inclination of mounting the optical disk, the optical disk has an inclination toward the objective lens, and coma aberration occurs in the converged light spot. Because of this coma aberration, if the number of apertures is increased, the converging performance is not improved. Accordingly, in order to prevent the coma aberration from increasing by increasing the number of apertures of the objective lens, it is attempted to heighten the density by reducing the thickness of the optical disk to 0.6 mm. When the thickness of the optical disk is reduced, however, a conventional optical disk cannot be reproduced by the objective lens for recording and reproducing this optical disk, and compatibility with the conventional optical disk is not assured.
To solve this problem, a bifocal optical head as shown in FIG. 24 is proposed. In FIG. 24, the radiation luminous flux emitted from a semiconductor laser 41 is collected by a collect lens 42 to be parallel light beam 43. This light beam 43 enters a polarized light beam splitter 44 as P polarized light. Hence, the light beam entering the polarized light beam splitter 44 almost completely passes through it, and is transformed into a nearly circularly polarized light by a quarter wavelength plate 45, and enters an objective lens 7 as the optical path is bent by a reflection mirror 46. A hologram 8 is formed on the inner circumference of the incident plane of the objective lens 7. This hologram is a blazed hologram, and higher order diffracted light can be suppressed. Accordingly, the light passing through the hologram 8 is divided into a primary diffracted light mainly diffracted by this hologram 8 and order 0 diffracted light not affected by diffraction. The primary diffracted light forms a light spot 9a focussed by the objective lens 7, and the order 0 diffracted light forms a light spot 9b focussed by the objective lens 7, together with the light passing through the outer circumference of the incident plane of the objective lens 7,on which hologram 8 is not formed. The light spot 9a is for reproducing an optical disk 10a of 1.2 mm in thickness, and the light spot 9b is for reproducing an optical disk 10b of 0.6 mm in thickness. FIG. 25A shows the state of converging the optical spot 9a on the information recording medium surface of the optical disk 10a, and FIG. 25B shows the state of converging the optical spot 9b on the information recording medium surface of the optical disk 10b. When reproducing the optical disk 10a of 1.2 mm in thickness, it is controlled so that the light spot 9a may be formed on the recording medium surface of the optical disk 10a, and reproducing the optical disk 10b of 0.6 mm in thickness, it is controlled so that the light spot 9b may be formed on the recording medium surface of the optical disk 10b. In this way, optical disks 10a, 10b differing in thickness can be recorded and reproduced by one objective lens 7. Incidentally, FIG. 24 shows the state of converging the light spot 9a on the optical disk 10a. Reflected light 47a or 47b (47b is not shown) reflected from the optical disk 10a or 10b passes again through the objective lens 7, hologram 8, reflection mirror 46, and quarter wavelength plate 45, and enters the polarized light beam splitter 44. If the optical disk is free from birefringence, the reflected light 47a or 47b becomes an S polarized light by the action of the quarter wavelength plate 45, and is reflected by the polarized light beam splitter 44, passes through an iris lens 48 and a cylindrical lens 49, and is received by a photo detector 50. The photo detector 50 is designed to detect the reproduced signal, and also detect the focus control signal by astigmatism method and the tracking control signal by phase difference method.
In this constitution, the 0.6 mm thick optical disk 10b is, for example, a high density optical disk, and is manufactured to be small in birefringence. This high density optical disk includes a two-layer disk type for reproducing two sides from one side through a layer of scores of microns, and it requires a greater quantity of light than CD reproduction of one side only because the quantity of reflected light declines. Moreover, to keep the S/N ratio in high frequency region, it is hard to decrease the quantity of light. Accordingly, in the optical head used in high density optical disk, a polarized light beam splitter is used in separation of reflected light and illumination light, and the loss of quantity of light due to separation is kept to a minimum. On the other hand, the 1.2 mm thick optical disk 10a is, for example, a CD, and those having a large birefringence over the standard are sold on market. In the CD not requiring such large quantity of light as in the high density optical disk, a half mirror is used for separation of reflected light and illumination light, and the loss of quantity of light due to separation is large, but it is not affected by birefringence, and therefore it is possible to cope with the optical disk 10a of large birefringence over the standard. In the common optical head for reproducing both high density optical disk and CD by one optical head, there is a contradictory problem between keeping of a sufficient quantity of light for high density optical disk and coping with a large birefringence for CD. So far, however, there is no optical disk capable of sufficiently solving this contradictory problem, and the polarized light beam splitter is used in the prior art, and when the birefringence of the optical disk 10a is large, most of the reflected light 47a passes through the polarized beam splitter 44, and the quantity of light reaching the photo detector 50 is lowered, and thereby reproduction is difficult.
In such conventional constitution, since the light from the light source and the reflected light from the optical disk are separated by using a polarized light beam splitter, it cannot cope with an optical disk large in birefringence. In particular, the CD has a large birefringence, and it may reach nearly half wavelength in commercial products. If the optical disk has no birefringence, the reflected light 47a or 47b enters the polarized light beam splitter 44 as S polarized light due to the action of the quarter wavelength plate , but if the optical disk has a birefringence of half wavelength, the reflected light 47a or 47b enters the polarized light beam splitter 44 as P polarized light, and it passes through to be fed back to the semiconductor laser 41. As a result, the reflected light does not reach the photo detector 50, and cannot be reproduced. If the birefringence of the optical disk is not as large as 1/2 wavelength, if very large, the quantity of light reaching the photo detector 50 is extremely lowered, and reproduction is difficult. Accordingly, the optical head for general CD is designed to separate the light from the light source and the reflected light from the optical disk by a half mirror. In the high density optical disk, however, in order to enhance the converging performance, the light beam 43 is increased in the ratio of light intensity in the surrounding to the light density in the center, and the light intake rate is small, and further due to drop of quantity of reflected light by two-layer disk and to keep the quantity of light for improvement of S/N ratio of high frequency components, it is hard to use the half mirror. Or, as in the prior art, in the separation by polarization using polarized light beam splitter, it cannot cope with the optical disk of large birefringence as mentioned above. The entire disclosure of U.S. Pat. No. 5,790,503, issued Aug. 4, 1998 is expressly incorporated by reference herein.